In accordance with the Release 7 third generation partnership project (3GPP) standards, wireless transmit/receive units (WTRUs) may be in either an idle state or a connected state. Based on the WTRU mobility and activity while in the connected state, a universal terrestrial radio access network (UTRAN) may direct the WTRU to transition between a number of sub-states: Cell_PCH, URA_PCH, Cell_FACH, and Cell_DCH states. User plane communication between the WTRU and the UTRAN is only possible while in Cell_FACH and Cell_DCH states. The Cell_DCH state is characterized by dedicated channels in both uplink and downlink. On the WTRU side, the Cell_DCH state corresponds to continuous transmission and reception and may be demanding on user power requirements. The Cell_FACH state does not use dedicated channels and thus allows better power consumption at the expense of a lower uplink and downlink throughput.
In pre-Release 8 3GPP standards, uplink communication is achieved through a random access channel (RACH) mapped to a physical random access channel (PRACH). The RACH is a contention-based channel and a power ramp-up procedure is used to acquire a channel and to adjust transmit power. An RACH is a shared channel used for an initial access to obtain dedicated resources or to transmit small amount of data. Because the RACH is shared and the access is random among WTRUs, there is a possibility of collision between two or more WTRUs trying to access the channel simultaneously.
In the Release 7 3GPP specifications, the RACH procedure has two stages: a channel acquisition stage using a slotted-ALOHA mechanism followed by an RACH message transmission stage. A WTRU that wants to access a channel, randomly selects a signature and transmits an RACH preamble to a Node B during a randomly selected access slot at a certain transmit power level. If the Node B detects the signature and if an associated resource is free, the Node B transmits a positive acknowledgement (ACK) on an acquisition indicator channel (AICH). After receiving an acquisition indicator (AI), (i.e., ACK), on the AICH, the WTRU transmit an RACH message. If the associated resource is unavailable, the Node B responds with a negative acknowledgement (NACK) on the AICH. This triggers a back-off mechanism at the WTRU. The WTRU starts a back-off timer Tbo1. After expiry of the timer a preamble ramping cycle count is incremented and the procedure starts again. This effectively restarts the RACH procedure at a later random time. If the RACH preamble from the WTRU is not detected at the Node B, no AI is transmitted on the AICH. If the WTRU fails to receive an AI after transmission of the RACH preamble, the WTRU tries again in a subsequent access slot with a randomly chosen signature and a higher transmit power, up to the maximum number of times.
Since the signature is chosen randomly from a list of available signatures and the RACH access procedure is anonymous, the Node B does not know which WTRU is accessing the channel until the Node B decodes the RACH message. Therefore, when two or more WTRUs happen to chose the same signature in the same access slot and one of them is detected by the Node B, the Node B will transmit an ACK. The WTRUs will all interpret this as a having acquired the channel and will access the channel simultaneously to transmit RACH messages. This causes a collision on the RACH messages. When a collision occurs, the RACH messages may not be decoded correctly. Collisions may be difficult to detect and incur additional delays.
The RACH procedure is divided between the medium access control (MAC) layer and the physical layer. The physical layer controls the preamble transmission, signature selection, access slot selection, and preamble transmit power. On the other hand, the MAC layer controls the interpretation of the AICH response, (i.e., ACK, NACK, or no response), as well as the start of the physical layer procedure. Transmission failure and successful completion of the MAC procedure are indicated individually for each logical channel, using primitives (CMAC-STATUS-Ind for the radio resource control (RRC) or MAC-STATUS-Ind for the radio link control (RLC)).
Recently, it has been proposed to modify the uplink transmission mechanism in Cell_FACH state by combining the RACH channel acquisition stage with an enhanced dedicated channel (E-DCH). The procedure is known as enhanced uplink for Cell_FACH and IDLE mode. The Node B would choose an E-DCH resource from a set of common E-DCH resources that are shared amongst all WTRUs. The Node B responds to a WTRU channel access request by assigning one of these resources. The WTRU then starts transmission over the assigned E-DCH transport channel.
This approach would allow larger data messages to be transmitted with lower latency than what is possible with the conventional RACH. In effect, the E-DCH will likely be used for a longer period of time for data transmission. This will increase the impact that the message collisions would have on the latencies perceived by a user and on the system's spectral efficiency.
As the channel acquisition phase is identical to that of the RACH mechanism, collisions are still possible. If a collision occurs, the transmission of an uplink MAC protocol data unit (PDU) would be unsuccessful. In the case of the transmission of an RRC message, if the first PDU fails during the collision resolution phase, the entire RRC message will fail transmission. In such a case, the WTRU would have to wait a long time for the message to be retransmitted (expiry of the RRC timers). Additionally, if an unacknowledged mode (UM) radio link control (RLC) PDU that carries one segment of an RLC service data unit (SDU) fails due to collision, the remaining segments of the RLC SDU are invalid due to the fact that the receiver can no longer reassemble the RLC SDU. These scenarios will result in long transmission delays in both the RRC and application level.
Therefore, methods to minimize the upper layer delays associated with collisions and methods to control WTRU behavior after a collision should be provided.